In recent years, in the field of digital high-speed wireless communication systems, in order to meet the demands of the market for improved efficiency of frequency band usage and high-speed data communications, high-efficiency multiplexing schemes, such as code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM), have been widely employed. Those schemes have a feature that a plurality of carriers and codes are multiplexed upon generation of a transmission signal to cause instantaneous power (peak factor or crest factor) which is much higher than average power of the transmission signal. This puts a hard limitation on design of a transmission signal amplifier (transmission power amplifier).
In order to transmit a distortion-free transmission signal, the transmission power amplifier provided in a wireless communication device is required to have linear input/output characteristics, ideally as indicated by T(x) of FIG. 2. However, the actual input/output characteristics of the power amplifier exhibit non-linearity in the high output region as indicated by E(x), resulting in saturation. The use of a non-linear region increases the radiation level outside a desired frequency band. For that reason, an average operating point of the power amplifier is lowered to ensure a sufficient backoff with respect to an output saturation point (i.e., a margin from the operating point to the saturation point of FIG. 2), to thereby use only a linear region so as to suppress the radiation level outside the desired band.
It is therefore necessary to provide a larger backoff as the instantaneous power becomes larger, which means that suppressing the instantaneous power contributes to high efficiency and power saving of the power amplifier. Suppressing the instantaneous power inevitably results in signal degradation, and hence a technology for suppressing the instantaneous power with minimum signal degradation has been sought after.
The related art for realizing this is a method involving power limitation using a window function as described in Japanese Unexamined Patent Application Publication (JP-A) No. 2007-306346 (Patent Document 1).
Further, another related art is a method described in Japanese Unexamined Patent Application Publication (JP-A) No. 2008-078943 (Patent Document 2).